1. Technical Field
The invention relates in general to an Asynchronous Transfer Mode (ATM) network wherein a connection is established between a source ATM device and a destination ATM device by multiple network switching nodes, and in particular to a method for routing loopback cells from the switch engine of one of the switching nodes located on the route used by this connection.
2. Description of the Related Art
The use of ATM switching nodes in an Internet Protocol (IP) network is an attractive solution since ATM hardware switches have been extensively studied and are widely available in many different architectures. When an ATM network connection is established from a source ATM device to a destination ATM device through multiple switching nodes, the incoming cells to a switching node are automatically routed to the next switching node of the connection. To achieve such automatic routing, each cell includes an ATM header along with its payload. An ATM protocol engine within the switching node identifies an incoming ATM cell using a lookup table. In the case of a valid cell (“valid” meaning belonging to an existing connection), the protocol engine performs traffic management functions (traffic policing, congestion management, priority management, etc.) and queues the cell in an appropriate queue. A scheduler using priority-based scheduling procedures selects queues from which cells are to be transmitted. The cells within the selected queues are then dequeued.
Prior to transmitting the dequeued cells, the protocol engine adds routing labels to the each cell including the switch routing label (SRL) and the protocol engine correlator (PEC). The resulting internal cell format used within the switching node will be referred to hereinafter as a “labeled cell.”
The SRL contains either an explicit identification of the destination blade or else a pointer to a translation table located in the switching device that includes the explicit destination blade identity. The PEC is a pointer used by the protocol engine of the output adapter to identify the connection. The protocol engine in the output destination adapter receives the cell from the switching device. Similarly to the input protocol engine, the output protocol engine identifies the incoming cell by performing a lookup function on the appended protocol engine correlator, performs traffic management functions, queues the cell in the appropriate queues, dequeues the cell under control of a scheduler, removes the appended labels, swaps the ATM label, and transmits the cell on the connection destination ATM port(s).
ATM standards have defined Operation And Maintenance (OAM) procedures. These procedures are based on particular cells that are identifiable as OAM cells in accordance with specified values encoded in the payload type indicator (PTI) field of the ATM cell header. Some OAM cells are called “loopback cells.” Loopback cells can be either segment loopback cells or end-to-end loopback cells and may optionally include in their payload a source and a destination address indication. The ITU-T I610 specifications define the procedures to be performed by network equipment when receiving OAM loopback cells. In particular ITU-T I610 specifications define the input adapter and the output adapter as the two loopback locations for a switching node. These specifications further describe the loopback condition algorithm using the cells parameters (source address, destination address, segment or end-to-end). By allowing cells to loopback on a connection path at various locations (input or output adapter of the various switching nodes on the connection path), these procedures ATM connection monitoring or problem determination and failure isolation within the ATM connection.
When OAM cells such as loopback cells are received by a switching node, the node typically processes the OAM cells by transmitting said cells to a dedicated processing resource such as a local processor that would perform requisite OAM procedures. However, the procedure is costly inasmuch as it requires incorporating microprocessors on the adapter card of the switching node whereas the non-OAM connection cells use ASIC modules which are data processing units specifically designed for the routing of the ATM cells.